You can find people that sells this unreleased songs on many social networks like reddit, twitter, instagram.
For Demi Lovato has a user on twitter called @Demiunreleased they used to manage a website with unreleased Demi Lovato tracks.
If you're lucky you can find them on Soundcloud for free. Some artists like Lana Del Rey for example, has a plenty of leaks and pre released songs on Youtube with other names like pseudonyms.
There's a website called leakth.is where you can find some leaked songs.